Superconducting filter apparatus and wireless receiving amplifier

ABSTRACT

Disclosed is a superconducting filter apparatus having a refrigerator for cooling the superconducting filter to cryogenic temperatures, a pilot signal generator for generating a pilot signal that is outside the pass band and inputting the pilot signal to the superconducting filter together with an antenna receive signal, and a discriminating unit for discriminating abnormality in the refrigerator. If the refrigerator malfunctions and temperature of the superconducting filter rises, the pass band of the superconducting filter shifts to the low-frequency side and crosses the frequency of the pilot signal. The pilot signal passes through the superconducting filter at this time. The discriminating unit discriminates abnormality in the refrigerator based upon the pilot signal contained in the output of the superconducting filter.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a superconducting filter apparatus usedat a base station in mobile communications, and to a wireless receivingamplifier having a superconducting filter. More particularly, theinvention relates to a superconducting filter apparatus that is capableof rapidly detecting an abnormality in a refrigerator, and a wirelessreceiving amplifier having a superconducting filter.

[0002] Generally, in order to obtain a steep cut-off characteristic in acommunications filter, the number of filter stages must be increased.However, a problem which arises is a commensurate increase in loss inthe pass band. Accordingly, note has been taken of the fact that asuperconductor has a resistance that is lower than that of ordinarymetals by two to three orders of magnitude, and a superconducting filterthat holds loss in the pass band to the minimum has been put intopractical use employing a superconductor as the conductor of the filter.Such a superconducting filter has become the focus of much attention inrecent years for the purpose of effectively utilizing frequency inmobile communications, increasing subscriber capacity and increasingbase-station coverage area, etc. A known example of a superconductingmaterial for a superconducting filter is YBCO (Y—Ba—Cu—O), which has acritical temperature (T_(c)) on the order of 90 K. This material is usedat a temperature T_(c) on the order of 70 K, which is a temperature atwhich characteristics stabilize.

[0003]FIG. 18 is a diagram showing the structure of a conventionalwireless receiving amplifier having a superconducting filter. Asuperconducting filter (SCF) 1 and low-noise amplifier (LNA) 2 aresecured to a cold head 4 and accommodated within a vacuum vessel 3. Thecold head 4 is cooled by a refrigerator 5. The superconducting filter 1and low-noise amplifier 2 are cooled by the refrigerator 5 via the coldhead 4 and operate at T_(c)=70 K. The vacuum vessel 3 and refrigerator 5are disposed in a case 6 in such a manner that outdoor installation ispossible. Terminals 7 a, 7 b and 8 a, 8 b provided on the case 6 andvacuum vessel 3 are connected by coaxial cables 9 a, 9 b, respectively,and terminal 7 b, superconducting filter 1, low-noise amplifier 2 andterminal 8 b are connected by a coaxial cable 9 c.

[0004] As shown in FIGS. 19(A), (B), the superconducting filter 1 has astructure in which a filter electrode 1 b and an n-stage (n=5 in theillustration) λ/2 resonator 1 c are patterned by YBCO film on an MgOsubstrate 1 a having a thickness t of 0.5 mm, and the filter is sealedin an aluminum-alloy package id. The package 1 d prevents leakage ofelectromagnetic field, thereby cooling the substrate 1 a uniformly. FIG.19(A) is a plan view in which an upper cover le of the package has beenremoved, and FIG. 19(B) is a sectional view taken along line AA in (A).Further, reference characters 1 f, 1 g represent coaxial connectors and1 h a ground formed by YBCO film having a thickness of 0.4 μm.

[0005] The electrical connections in the vacuum vessel are as shown inFIG. 20. For example, two channels of wireless receiving amplifiers areformed. Superconducting filters 1, 1′ exhibit a prescribed pass-bandcharacteristic if they are cooled to a cryogenic temperature of 70 K,and output pass-band components from among signals contained in receivesignals that enter from input terminals 7 b, 7 b′. Low-noise amplifiers(LNA) 2, 2′ amplify the signals that have passed through thesuperconducting filters 1, 1′, and the amplified signals are deliveredfrom output terminals 8 b, 8 b′. The low-noise amplifiers 2, 2′ have again characteristic and a noise figure characteristic shown in FIGS.21(A), (B). The solid line indicates the characteristic at ordinarytemperature (=230C) and the dashed line the characteristic at acryogenic temperature of 77 K. It will be understood that when acryogenic temperature is attained, the gain rises by 2 dB and the noisefigure declines. That is, it is preferred that the low-noise amplifiers2, 2′ be used at cryogenic temperatures rather than at ordinarytemperature.

[0006] Thus, the superconducting filter 1 is accommodated within thevacuum vessel 3 and operates upon being cooled to cryogenic temperatureof, e.g., T=70 K by the refrigerator 5. Further, if the low-noiseamplifier (LNA) 2 that amplifies the received signal to a prescribedlevel also is cooled to a cryogenic temperature, then the noise figurecan be reduced. In general, therefore, the low-noise amplifier is cooledat the same time as the superconducting filter 1. A signal received byan antenna (not shown) is input to the case 6 from the input terminal 7a via an antenna feeder, the signal propagates through the coaxialcables 9 a, 9 c, only a signal of the necessary frequency band isextracted by the superconducting filter 1, this signal is amplified to aprescribed signal level by the low-noise amplifier 2, and the resultantsignal is output from the output terminal 8 a.

[0007] In a mobile communications system, the wireless receivingamplifier shown in FIG. 18 is installed outdoors, namely on the roof ofa building, and hence is placed in a hostile environment of hightemperatures and humidity as occur in mid-summer, etc. While thusexposed to very harsh conditions, the wireless receiving amplifier isrequired to exhibit stable operating reliability for an extended periodof time of, e.g., tens of thousands of hours. However, since manysliding parts are used in the refrigerator 5, mechanical malfunction isa possibility. If the refrigerator 5 malfunctions, the temperature,which is being held at, e.g., T=70 K, naturally will rise and thesuperconducting filter 1 will no longer perform its original function.The result is communication failure. Accordingly, if the refrigeratordevelops a failure, it is necessary to have a function for detecting thefailure immediately and reporting the failure, or a function fordetecting the failure while it is still minor and reporting the same. Inthe prior art, there are arrangements in which refrigerator abnormalityis detected by measuring the temperature in the vacuum vessel andperforming monitoring to determine whether the temperature has exceededa set temperature. However, the apparatus involved is in large in sizeand of great weight. This does not conform to the requirement for anapparatus of smaller size and lighter weight.

SUMMARY OF THE INVENTION

[0008] Accordingly, an object of the present invention is to provide asuperconducting filter apparatus and wireless receiving amplifier inwhich refrigerator malfunction can be detected rapidly and reliably, andin which it is possible to achieve a reduction in size and weight.

[0009] Another object of the present invention is to provide asuperconducting filter apparatus and wireless receiving amplifier inwhich refrigerator malfunction can be detected as well as the extent ofthe malfunction.

[0010] A further object of the present invention is to so arrange itthat the occurrence of malfunction in either a refrigerator or low-noiseamplifier or in both the refrigerator and amplifier can be detectedreliably.

[0011] A superconducting filter apparatus according to the presentinvention comprises a superconducting filter that exhibits a prescribedpass-band characteristic when cooled to cryogenic temperatures; arefrigerator for cooling the superconducting filter to cryogenictemperatures; a pilot signal generator for generating a pilot signalthat is outside the pass band and inputting the pilot signal to thesuperconducting filter together with an antenna receive signal; and adiscriminating unit for discriminating abnormality in the refrigeratorbased upon the level of a pilot signal contained in a signal that isoutput from the superconducting filter. If the refrigerator develops amalfunction and temperature rises, the pass band of the superconductingfilter shifts to the low-frequency side, the frequency of the pilotsignal falls within the pass band of the superconducting filter and thepilot signal passes through the superconducting filter. Accordingly,refrigerator abnormality can be detected by monitoring whether the pilotsignal is contained in the signal that is output from thesuperconducting filter. The following effects can be expected inaccordance with the present invention:

[0012] (1) Refrigerator malfunction can be detected rapidly and,moreover, the superconducting filter apparatus can be reduced in sizeand weight.

[0013] (2) By providing the pilot signal generator in the vicinity ofthe receive antenna, e.g., by providing a pilot-signal radiating antennain the vicinity of the receive antenna, the pilot signal can be insertedinto the receive signal and input to the superconducting filter withoutloss.

[0014] (3) By inserting an isolator into an antenna feeder line, it canbe so arranged that the pilot signal will not be radiated into spacefrom the antenna even if it is reflected by the superconducting filter.As a result, it can be so arranged that the pilot signal will not becomeinterference with regard to other communication channels.

[0015] (4) The level of the pilot signal contained in the signal that isoutput from the superconducting filter can be detected and the extent ofa malfunction can be determined based upon the waveform of the detectedlevel, e.g., the rate of change in the level.

[0016] (5) Pilot signals of two waves having different frequencies aregenerated and input to the superconducting filter, the discriminatingunit detects the level of each pilot signal and the extent of amalfunction can be determined based upon the waveforms of the detectedlevels.

[0017] (6) It is possible to construct a wireless receiving amplifier byconnecting a low-noise amplifier to the superconducting filter andcooling both the superconducting filter and low-noise amplifier tocryogenic temperatures, thereby amplifying and outputting the signal,which passes through the superconducting filter, by the low-noiseamplifier.

[0018] A wireless receiving apparatus according to the present inventioncomprises: a superconducting filter that exhibits a prescribed pass-bandcharacteristic when cooled to cryogenic temperatures; a low-noiseamplifier for amplifying a signal that is output from thesuperconducting filter; a refrigerator for cooling the superconductingfilter and the low-noise amplifier to cryogenic temperatures; pilotsignal applying means for applying a pilot signal that is outside thepass band to a portion intermediate the superconducting filter andlow-noise amplifier; and a discriminating unit for detecting a declinein the level of the pilot signal contained in a signal that is outputfrom low-noise amplifier, deciding that the refrigerator is abnormalwhen the level of the pilot signal falls to a set level, and decidingthat the low-noise amplifier is abnormal when the level of the pilotsignal falls to a level other than the set level.

[0019] If the refrigerator (superconducting filter) and low-noiseamplifier are both normal, half of the power of the pilot signal appliedby the pilot signal applying means advances in the direction of thesuperconducting filter and the portion of the pilot signal thatcorresponds to the other half of the power advances in the direction ofthe low-noise amplifier. If the superconducting filter operatesnormally, the pilot signal is totally reflected and looped back in thedirection of the low-noise amplifier so that the total power of thepilot signal is input to the low-noise amplifier as a result. If thetemperature rises owing to refrigerator malfunction, on the other hand,the pilot signal is absorbed by the superconducting filter and isconverted to heat in its entirety. Hence the power that enters thelow-noise amplifier is halved. Accordingly, the level of the pilotsignal contained in the signal output from the low-noise amplifier whenthe refrigerator is normal differs from that when the refrigerator isabnormal. When a malfunction occurs, there is a prescribed level drop incomparison with the time of normal operation. Thus, a drop in the levelof the pilot signal contained in the signal output from the low-noiseamplifier is monitored, the refrigerator is judged to be abnormal whenthe level of the pilot signal falls to a prescribed level, and thelow-noise amplifier is judged to be abnormal when the level of the pilotsignal falls to a level other than the prescribed level.

[0020] Further, a separate pilot signal is input to the superconductingfilter and refrigerator abnormality is detected based upon the detectedlevel of the pilot contained in the signal output from the low-noiseamplifier. If this arrangement is adopted, refrigerator malfunction canbe detected reliably and, moreover, it is possible to reliablydiscriminate abnormality of the low-noise amplifier based upon a drop inthe reception level of the pilot signal applied to the portionintermediate the superconducting filter and low-noise amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a diagram illustrating the structure of a wirelessreceiving amplifier according to the present invention;

[0022]FIG. 2 is a pass characteristic of a superconducting filter;

[0023]FIG. 3 is a diagram useful in describing temperature dependence ofthe pass characteristic and loss characteristic of the superconductingfilter;

[0024]FIG. 4 is a time waveform diagram of pilot detected level at thetime of a rise in temperature;

[0025]FIG. 5 is a diagram useful in describing the relationship betweenextent of refrigerator malfunction and the time waveform of the pilotdetected level;

[0026]FIG. 6 is a diagram showing the structure of a second embodimentof the present invention;

[0027]FIG. 7 is a diagram showing the structure of a third embodiment ofthe present invention;

[0028]FIG. 8 is a diagram showing the structure of a fourth embodimentof the present invention;

[0029]FIG. 9 is a diagram useful in describing a pass characteristic ina case where the extent of refrigerator malfunction is detected usingpilot signals of two waves;

[0030]FIG. 10 shows time waveforms of detected level in a case where theextent of refrigerator malfunction is detected using pilot signals oftwo waves;

[0031]FIG. 11 is a diagram showing the structure of a fifth embodimentof the present invention;

[0032]FIG. 12 is a diagram useful in describing the structure andoperation of a directional coupler;

[0033]FIG. 13 is a diagram useful in describing a pass characteristicS21/reflection characteristic S11 of a superconducting filter atcryogenic temperatures and at high temperatures;

[0034]FIG. 14 is a diagram useful in describing the principle offault-location detection according to a fifth embodiment;

[0035]FIG. 15 is a diagram showing the structure of a sixth embodimentof the present invention;

[0036]FIG. 16 is a table useful in describing correspondence betweenpilot detection levels and fault locations;

[0037]FIG. 17 is a diagram showing the structure of a superconductingfilter apparatus in which a low-noise amplifier is installed outside acase;

[0038]FIG. 18 is a diagram showing the structure of a conventionalwireless receiving amplifier having a superconducting filter;

[0039]FIG. 19 is a diagram useful in describing the superconductingfilter;

[0040]FIG. 20 is a diagram showing the electrical connections in avacuum vessel; and

[0041]FIG. 21 shows a gain characteristic and noise figurecharacteristic of the low-noise amplifier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] (A) First Embodiment

[0043] (a) Overall Configuration

[0044]FIG. 1 is a diagram showing the structure of a wireless receivingamplifier according to the present invention. A superconducting filter(SCF) 11 and low-noise amplifier (LNA) 12 are secured to a cold head 14and accommodated within a vacuum vessel 13. The cold head 14 is cooledby a refrigerator 15. The superconducting filter 11 and low-noiseamplifier 12 are cooled by the refrigerator 15 via the cold head 14 andoperate at Tc=70 K. The vacuum vessel 13 and refrigerator 15 aredisposed in a case 16 in such a manner that outdoor installation ispossible. Terminals 17 a, 17 b and 18 a, 18 b provided on the case 16and vacuum vessel 13 are connected by coaxial cables 19 a, 19 b,respectively, and terminal 17 b, superconducting filter 11, low-noiseamplifier 12 and terminal 18 b are connected by a coaxial cable 19 c.

[0045] A receive antenna 32 is connected to input terminal 17 a of thecase 16 via an antenna feeder 31, and a signal that has been receivedfrom the antenna is input to the superconducting filter 11 via the inputterminal 17 a. A pilot signal generator 33 generates a pilot signal, andthe pilot signal is superimposed upon the antenna receive signal via asignal coupler 34. Accordingly, the pilot signal is input to thesuperconducting filter together with the antenna receive signal.Frequency f_(c) of the pilot signal is a frequency that is outside thepass frequency of the superconducting filter 11 at 70 K.

[0046] A pilot signal detector 21 is connected to output terminal 18 aof the case 16, whether the signal output from the low-noise amplifier12 contains the pilot signal is monitored and the level thereof isdetected. The pilot signal detector 21 has a directional coupler 21 a inwhich the part of the input signal is loaded, a bandpass filter 21 bhaving a center frequency for passing the pilot signal, and a leveldetector 21 c for detecting the level of the pilot signal based upon theoutput of the bandpass filter.

[0047] (b) Principle of the Present Invention

[0048] The superconducting filter 11 exhibits a prescribed passcharacteristic S21 when it is cooled to a cryogenic temperature of 70 K.FIG. 2 shows an example of the pass characteristic of thesuperconducting filter 11 at T₀=70 K. The superconducting filter has apass band of 1950 to 1970 MHz. The superconducting filter 11 operatesbelow the critical temperature (T_(c)). If the temperature is raised asfollows: T=T₀→T₁→T₂ (T₀<T₁<T₂), as shown in FIG. 3(A), the centerfrequency f₀ of the filter pass band diminishes as follows: f₀₀→f₀₁→f₀₂,and insertion loss Loss increases. The closer the temperature approachesthe critical temperature T_(c), the greater the rate of change. As aresult, the pass characteristic S21 of the superconducting filter 11varies depending upon temperature, as shown in FIG. 3(B). In actuality,since the low-noise amplifier 12 is connected immediately following thesuperconducting filter 11, the overall pass characteristic variesdepending upon temperature, as shown in FIG. 3(C), as a result of thesignal being amplified by the gain of the low-noise amplifier 12.

[0049] Accordingly, a pilot signal having a frequency f_(c) outside thefilter pass band at T=T₀ (=70 K) is input to the superconducting filter11. However, the frequency f_(c) of the pilot signal is set so as to belower than the pass-band frequency. Thus, at T=T₀, the pilot signalfrequency f_(c) is outside the pass band of the superconducting filter11 and therefore the pilot signal Sfc is reflected by the filter portionand does not enter the low-noise amplifier 12.

[0050] If the temperature T rises owing to failure of the refrigerator15, however, the pass band of the superconducting filter 11 begins toshift to the low-frequency side, a shown in FIG. 3(C), and the pilotfrequency f_(c) gradually falls within the pass band of the filter 11.As a result, the pilot signal Sfc begins to pass through the filter 11(time t=t₁). As the temperature rises further, the amount of the pilotsignal passed gradually increases until the pilot frequency f_(c) fallsentirely within the pass band of the superconducting filter 11 (t=t₂).If the pilot frequency f_(c) falls completely within the pass band, theamount of the pilot signal passed becomes substantially constant. If thetemperature rises further and the pilot frequency f_(c) begins to fallwithin the attenuation region of the superconducting filter (t=t₃), theamount of the pilot signal passed gradually decreases. When the pilotfrequency f_(c) eventually falls outside the pass band (t=t₄), theamount of the signal passed becomes zero. As a result, the level of thepilot signal that enters the pilot signal detector 21 takes on awaveform of the kind shown in FIG. 4 with the passage of time.

[0051] Thus, if a threshold level L_(TH) is decided and this level isexceeded, then the pilot signal detector 21 decides that the frequencycharacteristic (pass characteristic) of the superconducting filter 11has changed. In other words, if the detected level exceeds the thresholdlevel L_(TH), the pilot signal detector 21 decides that the refrigerator15 has malfunctioned, thereby allowing a rise in temperature, andoutputs an alarm signal ALM.

[0052] (C) Detection of Refrigerator Malfunction

[0053] The superconducting filter 11 and low-noise amplifier 12 aresecured to the cold head 14 and held in vacuum within the vacuum vessel13 so as to be shielded against atmospheric temperature. Thesuperconducting filter 11 is obtained by, e.g., forming a filter patternhaving a pass region of 1920 to 1940 MHz on a magnesium oxide (MgO)substrate and consists of nine stages of a YBCO superconductor for whichT_(c)=90 K holds. The superconducting filter 11 is accommodated in ametal package having a size on the order of 50×50×15 mm and is cooleduniformly. At an operating temperature T₀=70 K, loss of less than 0.1 dBin the pass region can be achieved. The refrigerator 15 is supplied withpower from the outside and is driven and controlled by a driving powersupply and temperature regulator (not shown) in such a manner that afilter operating temperature of 70 K will be obtained. This equipment isaccommodated in the case 16 having a size on the order of 500×500×300,and the case is installed outdoors, as on the roof of a building.Consequently, the internal temperature can rise to 60 to 80° C., as on asummer afternoon. It is required that the refrigerator 15 operate stablyfor an extended period of time even when subjected to harsh conditions.

[0054] The signal that has been received from the antenna enters fromthe input terminal 17 a, only a signal of the desired frequency bandpasses through the superconducting filter 11, the passed signal isamplified by the low-noise amplifier 12, and the amplified signal isoutput from the output terminal 18 a and sent to the next stage via thepilot signal detector 21. Accordingly, if a pilot signal outside thefilter band (e.g., pilot frequency f_(c)=1900 MHz) is impressed upon thereceive signal and the resultant signal is input to the input terminal17 a, then the pilot frequency f_(c) will be outside the pass band ofthe filter in the case of normal operation (T=T₀) and, hence, the signalwill be reflected by the filter portion. As a result, the pilot signalSfc will not enter the low-noise amplifier 12 and will not be containedin the output signal of the low-noise amplifier.

[0055] However, if the refrigerator 15 malfunctions and the temperatureof the superconducting filter portion rises, the frequencycharacteristic begins to shift to the low-frequency side and the pilotsignal begins to pass through the superconducting filter 11. The pilotsignal that has passed through the filter is amplified by the low-noiseamplifier 12 and the amplified signal is sent to the pilot signaldetector 21. The latter extracts only the pilot-signal component andsamples the level of the pilot signal at fixed time intervals. If therefrigerator 15 malfunctions, a time waveform of the detected levelshown in FIG. 4 is obtained. If the detected level exceeds the thresholdvalue, therefore, the pilot signal detector 21 outputs the alarm signalALM, which indicates refrigerator malfunction.

[0056] Thus, as set forth above, the pilot signal detector 21 samplesand records the detected level of the pilot signal at fixed intervals(see FIG. 4). The rate of change of the detected level at the times ofthe rise and fall of the time waveform and the length of time thethreshold level L_(TH) is exceeded differ in dependence upon the rate oftemperature rise. That is, in the event of a serious refrigeratormalfunction, the rate of the rise in temperature increases, the risingedge and falling edge of the detected-level time waveform become steep,as indicated by Case B in FIG. 5, and the length of time the thresholdlevel L_(TH) is exceeded diminishes. In the case of a minor refrigeratormalfunction, on the other hand, the rate of the rise in temperaturedecreases, the rising edge and falling edge of the detected-level timewaveform become less steep, as indicated by Case C in FIG. 5, and thelength of time the threshold level L_(TH) is exceeded increases. In thecase of moderate refrigerator malfunction, the slope of the rising edgeand falling edge of the detected-level time waveform and the length oftime the threshold level L_(TH) is exceeded become intermediate those ofCases B and C, as indicated by Case A in FIG. 5. Thus, if a mechanismthat identifies the waveforms of Cases A to C is mounted on the pilotsignal detector 21, the extent of the refrigerator malfunction can beascertained. In other words, the extent of the a failure can beclassified as follows: Case B=serious, Case C=minor, Case A=average, andan order of priority regarding the dispatch of a serviceman can beassigned in accordance with the extent of failure. This is useful interms of providing specific feedback for troubleshooting.

[0057] (B) Second Embodiment

[0058]FIG. 6 is a diagram showing the structure of a second embodimentof the present invention. This embodiment has a preferable arrangementfor inserting a pilot signal into the signal received by the antenna.Components identical with those of the first embodiment in FIG. 1 aredesignated by like reference characters.

[0059] The pilot signal can be impressed upon the antenna receive signalusing a coupler or the like, as in the first embodiment. However, lossfrom ahead of the low-noise amplifier 12 increases, the noise figure(NF) increases and the advantage of cooling the low-noise amplifier 12cannot be fully exploited. Accordingly, the pilot signal generator 33 isplaced in the vicinity of the receive antenna 32, as shown in FIG. 6.The pilot signal generator 33 has an oscillator 33 a for generating apilot signal having the frequency f_(c) outside the pass band of thesuperconducting filter 11, and a directional antenna 33 b for radiatingthe pilot signal toward the receive antenna 32. The antenna 33 btherefore is pointed toward the receive antenna 32 and is placed in thevicinity thereof. If the oscillator 33 a is caused to oscillate underthese conditions, the pilot signal is radiated toward the receiveantenna 32, and the latter receives the pilot signal from a mobilestation and inputs the pilot signal to the signal input terminal 17 avia the antenna feeder 31. As a result, the pilot signal can besuperimposed upon the receive signal from the mobile station without theinclusion of needless loss, as in the case of the coupler, and theresultant signal is input to the input terminal 17 a. Thus, anabnormality in the refrigerator can be detected without increasing thenoise figure (NF).

[0060] (C) Third Embodiment

[0061]FIG. 7 is a diagram showing the structure of a third embodiment ofthe present invention. This arrangement is the same as that of thesecond embodiment in FIG. 6 except for the fact that an isolator 35 isprovided between the receive antenna 32 and the input terminal 17 a ofthe case 16. Identical components are designated by like referencecharacters.

[0062] In a case where the refrigerator 15 is operating normally, thepilot signal outside the pass band of the superconducting filter 11 isalmost totally reflected by the filter portion and the reflected pilotsignal radiates in reverse from the antenna 32. In a case where theantenna 32 possesses directivity and has gain, the reflected signal isradiated with a higher signal level at this time and may constituteinterference with respect to other communication channels. Accordingly,in the third embodiment, the isolator 35 is inserted into the antennafeeder 31 to isolate the pilot signal that is reflected from thesuperconducting filter 11. In accordance with the third embodiment, areflected pilot signal will not be radiated from the antenna and willnot have an adverse effect upon other communication channels.

[0063] (D) Fourth Embodiment

[0064]FIG. 8 is a diagram showing the structure of a fourth embodimentof the present invention. This embodiment has an arrangement forinserting first and second pilot signals into the signal received by theantenna. Components identical with those of the second embodiment inFIG. 6 are designated by like reference characters. The pilot signalgenerator 33 disposed in the proximity of the receive antenna 32 has anoscillator 33 c for generating a first pilot signal Sfc₁ of frequencyfc₁, an oscillator 33 d for generating a second pilot signal Sfc₂ offrequency fc₂ (>fc₁), a combiner 33 e for combining the first and secondpilot signals Sfc₁, Sfc₂, and an antenna 33 f for radiating the firstand second pilot signals toward the receive antenna 32.

[0065] If the refrigerating capability of the refrigerator declinesowing to prolonged use (minor malfunction), the operating temperature ofthe superconducting filter 11 will rise above T₀ (=70 K) owing to a risein the ambient temperature of the refrigerator. However, if night fallsand the ambient temperature of the refrigerator falls to roomtemperature, the operating temperature of the superconducting filter 11will return to T₀ and the filter will operate normally. In the case of aserious malfunction, on the other hand, the operating temperature of thesuperconducting filter 11 rises above and stays above T₀ (=70 K) and thesuperconducting filter 11 can no longer operate normally. If the extentof such a malfunction can be detected, then it will be possible to takemeasures that conform to the extent of the malfunction.

[0066] Accordingly, as shown in FIG. 9(A), the two pilot signals Sfc₁,Sfc₂ of the frequencies fc₁, fc₂, respectively, that are outside thepass band of the superconducting filter 11 for which T=T₀ holds areinput to the superconducting filter 11 together with the receive signal,and the levels of the pilot signals Sfc₁, Sfc₂ are detected by the pilotsignal detector 21 to discriminate the extent of refrigeratormalfunction.

[0067] If the refrigerator malfunction is minor, as when there is atemporary decline in refrigerating performance, the pass characteristic(frequency characteristic) of the superconducting filter 11 shiftstemporarily to the low-frequency side owing to the temperature rise.Since the temperature returns to normal, however, the passcharacteristic also returns to normal. In other words, owing to atemporary rise in temperature, the pass characteristic of thesuperconducting filter 11 is as indicated by the dashed line in FIG.9(B). Thereafter, the characteristic returns to the pass characteristicof the solid line owing to the return to normal temperature. When thetemperature rises, therefore, first the pilot signal Sfc₂ of frequencyfc₂ is detected, a shown in FIG. 10(A), then the pilot signal Sfc₁ offrequency fc₁ is detected. When the temperature returns to normal, onthe other hand, first the pilot signal Sfc₁ of frequency fc₁ stops beingdetected, then the pilot signal Sfc₂ of frequency fc₂ stops beingdetected. If the temperature rise is slight, the pass characteristic ofthe superconducting filter 11 becomes as indicated by the dashed line inFIG. 9(C), the pilot signal Sfc₁ of frequency fc₁ is not detected andonly the pilot signal Sfc₂ is detected. The time waveform of thedetected level becomes as shown in FIG. 10(B).

[0068] If the malfunction is serious, on the other hand, the passcharacteristic (frequency characteristic) of the superconducting filter11 becomes as indicated by the dashed line in FIG. 9(D) owing to a risein temperature and, hence, frequency shifts to the low-frequency sidefrom the frequencies fc₁, fc₂ and remains there. When the temperaturerises, therefore, first the pilot signal Sfc₂ of frequency fc₂ isdetected, as shown in FIG. 10(C), then the pilot signal Sfc₁ offrequency fc₁ is detected. As the temperature rises further, first thepilot signal Sfc₂ of frequency fc₂ stops being detected, then the pilotsignal Sfc₁ of frequency fc₁ stops being detected.

[0069] The pilot signal detector 21 extracts the components of the pilotsignals Sfc₁, Sfc₂, samples the levels of each of the pilot signals atfixed time intervals and detects the extent of the malfunction basedupon the time waveform of the detected level of each pilot signal. Ifthis arrangement is adopted, an order of priority regarding the dispatchof a serviceman can be assigned in accordance with the extent offailure. This is useful in terms of providing specific feedback fortroubleshooting.

[0070] (E) Fifth Embodiment

[0071]FIG. 11 is a diagram showing the structure of a fifth embodimentof the present invention. This embodiment detects refrigeratormalfunction and malfunction of the low-noise amplifier. Componentsidentical with those of the first embodiment in FIG. 1 are designated bylike reference characters. In the fifth embodiment, a directionalcoupler 41 is provided intermediate the superconducting filter 11 andlow-noise amplifier 12, and the pilot signal that is output from a pilotsignal generator 42 is superimposed upon the output signal of thesuperconducting filter 11 via this directional coupler. The frequencyf_(L) of the pilot signal is outside the pass band of thesuperconducting filter 11. By way of example, f_(L)=2000 MHz holds. Thedirectional coupler 41, which has the structure shown in FIG. 12,couples the pilot signal, which is output from the pilot signalgenerator (oscillator) 42, in such a manner that the signal flows in thedirection of the superconducting filter 11 and in the direction of thelow-noise amplifier 12.

[0072] When operation is normal, an applied pilot signal Sf_(L) isamplified by the low-noise amplifier 12 and the amplified signal isoutput from the output terminal 18 a together with the antenna receivesignal. A pilot signal amplifier 43 detects the level of the pilotsignal and checks the low-noise amplifier 12 for abnormality based uponthe level of the pilot signal. That is, since the detected level of thepilot signal Sf_(L) falls if the low-noise amplifier 12 malfunctions,the pilot signal amplifier 43 checks the low-noise amplifier forabnormality based upon the level of the pilot signal.

[0073] If the superconducting filter 11 is operating normally when apilot signal outside the pass band is input to the superconductingfilter 11, the pilot signal is totally reflected by the superconductingfilter 11, as illustrated in FIG. 14(A), since return loss [see S11(T=70 K)] is approximately 0 dB, as shown in FIG. 13. However, if therefrigerator malfunctions, the temperature exceeds the criticaltemperature and the superconducting state can no longer be achieved, thereturn cross [see S11 (T=300 K)] becomes −10 dB to −20 dB. As a result,the pilot signal is reflected only by 10 to 1% by the superconductingfilter 11, as shown in FIG. 14(B), and the remaining 90 to 99% of thesignal is absorbed within the superconducting filter and converted toheat.

[0074] Thus, if the superconducting filter 11 is operating normally, thepilot signal is totally reflected, even though it attempts to flow intothe superconducting filter 11 from the coupler 41, and signal flows tothe side of the low-noise amplifier 12 and the total power of the pilotsignal flows into the low-noise amplifier 12. If the refrigerator 15malfunctions and the critical temperature is exceeded, however, thepilot signal is not reflected and flows into the superconducting filter11. As a consequence, the power of the pilot signal that flows into thelow-noise amplifier 12 becomes half of that at the time of normaloperation. That is, since the pilot signal flows into thesuperconducting filter 11 and low-noise amplifier 12 by being split at aratio of 5:5 by the coupler 41, the detected level of the pilot signalin the pilot signal amplifier 43 falls by 3 dB at the time ofmalfunction. It should be noted that since the gain of the low-noiseamplifier 12 also declines when temperature rises, in actuality thelevel falls by 5 dB inclusive of the fall in gain. Since the fall inlevel is a known value owing to the characteristics of the coupler andlow-noise amplifier, this will be referred to as L_(D) (dB) below.

[0075] The pilot signal amplifier 43 constantly determines whether thelevel of the pilot signal has fallen by L_(D) (dB). For example, if thelevel falls by L_(D) (dB), the pilot signal amplifier 43 outputs thealarm signal ALM on the grounds that the refrigerator 15 hasmalfunctioned. If a fall in level in excess of L_(D) (dB) occurs, thepilot signal amplifier 43 outputs the alarm signal on the grounds thatthe low-noise amplifier 12 has malfunctioned.

[0076] (F) Sixth Embodiment

[0077]FIG. 15 is a diagram illustrating the structure of a sixthembodiment of the present invention. This embodiment is obtained bycombining the second and fifth embodiments to make it possible to detectrefrigerator malfunction and malfunction of the low-noise amplifier in ahighly precise manner. Components identical with those of the second andfifth embodiments are designated by like reference characters. As pilotsignals, the sixth embodiment uses {circle over (1)} the pilot signalSf_(c) of frequency f_(c) (=1900 MHz), which is for detectingrefrigerator malfunction, generated by the pilot signal generator 33,and {circle over (2)} the pilot signal Sf_(L) of frequency f_(L) (=2000MHz), which is for detecting malfunction of the low-noise amplifier,generated by the pilot signal generator 42.

[0078] The antenna receive signal, pilot signal Sf_(c) of frequencyf_(c) and the pilot signal Sf_(L) of frequency f_(L) are each outputfrom the output terminal 18 a and pass through the pilot signal detector21 of frequency f_(c) and pilot signal amplifier 43 of frequency f_(L)in the order mentioned. The detectors 21, 43 detect the levels of thepilot signals of frequencies f_(c), f_(L), respectively, and input thedetected levels to a fault-location discriminator 51. The latterdiscriminates fault location in accordance with FIG. 16. Specifically,the fault-location discriminator 51 decides that the refrigerator 15 isnormal if the pilot signal Sf_(c) of frequency f_(c) is not detected anddecides that the refrigerator 15 has malfunctioned if the pilot signalSf_(c) of frequency f_(c) is detected. Further, the fault-locationdiscriminator 51 (1) decides that the refrigerator and amplifier arenormal if the detected level of the pilot signal Sf_(L) is normal; (2)decides that the low-noise amplifier 12 has malfunctioned if therefrigerator is normal and the detected level of the pilot signal Sf_(L)falls by L_(D) (dB); (3) decides that the low-noise amplifier 12 hasmalfunctioned if the refrigerator is normal and the detected level ofthe pilot signal Sf_(L) falls by any decibel amount; (4) decides thatthe low-noise amplifier 12 is normal if the refrigerator is abnormal andthe detected level of the pilot signal Sf_(L) falls by L_(D) (dB); and(5) decides that the low-noise amplifier 12 also is abnormal if therefrigerator is abnormal and the detected level of the pilot signal SfLfalls by more than L_(D) (dB).

[0079] Thus, in accordance with the sixth embodiment, it is possible toreliably detect whether the refrigerator has malfunctioned, whether thelow-noise amplifier has malfunctioned or whether both havemalfunctioned. This is advantageous in that it will suffice to replacethe minimum number of faulty parts. In particular, if the level falls byL_(D) (dB), it is possible to specify the cause, namely whether thelevel has fallen only by L_(D) (dB) by chance malfunction of thelow-noise amplifier or whether the level has fallen by L_(D) (dB) owingto malfunction of the refrigerator.

[0080] (G) Seventh Embodiment

[0081] In the foregoing, the low-noise amplifier 12 is accommodatedwithin a vacuum vessel and is cooled together with the superconductingfilter 11. In each embodiment, however, the low-noise amplifier 12 neednot necessarily be cooled and can be provided externally of the case 16.FIG. 17 is an example in which the low-noise amplifier 12 of the secondembodiment (see FIG. 6) is placed outside the case 16. Reference numeral50 denotes a superconducting filter apparatus.

[0082] Thus, in accordance with the present invention, it is possible todiscriminate and report whether a refrigerator has malfunctioned, thenature of the malfunction and the extent of the malfunction, and it ispossible to minimize circumstances in which mobile communication fails.

[0083] Further, in accordance with the present invention, refrigeratorfailure can be detected quickly and reliably. Moreover, since pilotsignal inserting means and pilot signal generating means need only beprovided as hardware, the superconducting filter apparatus and wirelessreceiving amplifier can be reduced in size and weight.

[0084] Further, in accordance with the present invention, whether therefrigerator or low-noise amplifier has failed, or whether both havefailed, can be detected in a reliable manner.

What is claimed is:
 1. A superconducting filter apparatus comprising: asuperconducting filter that exhibits a prescribed pass-bandcharacteristic when cooled to cryogenic temperatures; a refrigerator forcooling said superconducting filter to cryogenic temperatures; a pilotsignal generator for generating a pilot signal that is outside said passband and inputting said pilot signal to the superconducting filtertogether with an antenna receive signal; and a discriminating unit fordiscriminating abnormality in the refrigerator based upon the level ofthe pilot signal contained in a signal that is output from thesuperconducting filter.
 2. A superconducting filter apparatus accordingto claim 1, wherein said pilot signal generator is provided in thevicinity of a receive antenna.
 3. A superconducting filter apparatusaccording to claim 1, wherein said pilot signal generator has a pilotsignal radiating antenna, said radiating antenna being provided in thevicinity of said receive antenna.
 4. A superconducting filter apparatusaccording to claim 2, wherein an isolator is inserted into an antennafeeder line.
 5. A superconducting filter apparatus according to claim 1,wherein said discriminating unit detects the level of the pilot signalcontained in the signal that is output from the superconducting filterand judges extent of an abnormality based upon the waveform of thedetected level.
 6. A superconducting filter apparatus according to claim1, wherein said pilot signal generator generates two waves of pilotsignals having different frequencies and inputs the pilot signals tosaid superconducting filter, and said discriminating unit detects thelevel of each pilot signal and judges extent of an abnormality basedupon the waveforms of each of the detected levels.
 7. A wirelessreceiving amplifier for amplifying a signal of a prescribed band in asignal received by an antenna and outputting the amplified signal,comprising: a superconducting filter that exhibits a prescribedpass-band characteristic when cooled to cryogenic temperatures; alow-noise amplifier for amplifying a signal that is output from thesuperconducting filter; a refrigerator for cooling said superconductingfilter and low-noise amplifier to cryogenic temperatures; a pilot signalgenerator for generating a pilot signal that is outside said pass bandand inputting said pilot signal to the superconducting filter togetherwith an antenna receive signal; and a discriminating unit fordiscriminating abnormality in the refrigerator based upon the level ofthe pilot signal contained in a signal that is output from the low-noiseamplifier.
 8. A wireless receiving amplifier according to claim 9,wherein said pilot signal generator has a pilot signal radiatingantenna, said radiating antenna being provided in the vicinity of saidreceive antenna.
 9. A wireless receiving amplifier according to claim 8,wherein an isolator is inserted into an antenna feeder line.
 10. Awireless receiving amplifier according to claim 7, wherein saiddiscriminating unit detects the level of the pilot signal contained inthe signal that is output from the low-noise amplifier and judges extentof an abnormality based upon the waveform of the detected level.
 11. Awireless receiving amplifier according to claim 7, wherein said pilotsignal generator generates two waves of pilot signals having differentfrequencies and inputs the pilot signals to said superconducting filter,and said discriminating unit detects the level of each pilot signalcontained in the signal that is output from the low-noise amplifier andjudges extent of an abnormality based upon the waveforms of each of thedetected levels.
 12. A wireless receiving amplifier for amplifying asignal of a prescribed band in a signal received by an antenna andoutputting the amplified signal, comprising: a superconducting filterthat exhibits a prescribed pass-band characteristic when cooled tocryogenic temperatures; a low-noise amplifier for amplifying a signalthat is output from the superconducting filter; a refrigerator forcooling said superconducting filter and low-noise amplifier to cryogenictemperatures; pilot signal applying means for applying a pilot signalthat is outside said pass band to a portion intermediate thesuperconducting filter and low-noise amplifier; and a discriminatingunit for detecting a decline in the level of the pilot signal containedin a signal that is output from the low-noise amplifier, deciding thatthe refrigerator is abnormal when the level of the pilot signal falls toa predetermined level, and deciding that the low-noise amplifier isabnormal when the level of the pilot signal falls to a level other thanthe predetermined level.
 13. A wireless receiving amplifier foramplifying a signal of a prescribed band in a signal received by anantenna and outputting the amplified signal, comprising: asuperconducting filter that exhibits a prescribed pass-bandcharacteristic when cooled to cryogenic temperatures; a low-noiseamplifier for amplifying a signal that is output from thesuperconducting filter; a refrigerator for cooling said superconductingfilter and low-noise amplifier to cryogenic temperatures; a pilot signalinput unit for generating a first pilot signal that is outside said passband and inputting this pilot signal to the superconducting filtertogether with an antenna receive signal; a pilot signal applying unitfor applying a second pilot signal that is outside said pass band to aportion intermediate the superconducting filter and low-noise amplifier;and a discriminating unit for discriminating abnormality in therefrigerator based upon detected level of the first pilot signalcontained in a signal that is output from the low-noise amplifier, anddiscriminating abnormality in the low-noise amplifier based upon adecline in the level of the second pilot signal.